Method and apparatus for measuring quantities relating to a persons cardiac activity

ABSTRACT

A method and an apparatus for measuring quantities relating to a person&#39;s cardiac activity, such as for example quantities proportional to heart rate, cardiac stroke volume and cardiac output. A person is positioned on a measuring support (S), so that a blood flow pulse caused by a stroke of heart is matched by a change in a person&#39;s weight (G). Said change of weight is recorded and the recorded changes of weight are used for deriving information about a person&#39;s cardiac activity, such as for example about stroke volume and cardiac output as well as heart rate.

The present invention relates to a method and apparatus for measuringquantities relating to the cardiac activity and physical condition of aperson, including quantities correlating to the stroke volume andcardiac output which relate to the heart rate and a person's weight andcardiac blood circulation, and for using the measuring results forcalculating an index representing the physical condition of a person.

The cardiac pulse rate or heart rate HR, whose unit is pulses per minuteas measured in conditions of rest and various stresses, provides e.g. arepresentation of the physical condition of a person being examined. HRincreases not only as a result of mental and physical stress but alsovarious illnesses, for example fever. At rest, HR of a person in a goodcondition is typically lower than HR of a person in a poor condition.

In the evaluation of the cardiac activity, the measurement of HR onlyprovides highly superficial information. It is also important to knowthe cardiac stroke volume SV, whose unit is liter, and the cardiacoutput CO, whose unit is liters per minute. CO is obtained for exampleby multiplying HR and the average SV with each other.

It is prior known to measure HR by recording electric cardiac activity,a so-called ECG-signal, used for identifying e.g. so-calledQRS-complexes. The heart rate is obtained by measuring the time betweensuccessive QRS-complexes, whereby the inverse value of said timeindicates HR. Devices based on this principle are manufactured forathletes and fitness enthusiasts e.g. by Polar Electro OY, Finland. Adrawback in these devices is the necessity of attaching electrodes tothe body. Neither does the ECG-signal provide any information about SVor CO.

It is also prior known to measure HR by utilizing variations ofelectrical impedance and light transmission. Light transmission can bemeasured e.g. from the earlobe and finger. An HR measuring device forathletes based on light transmission is manufactured e.g. by Casio,Japan. A drawback in this type of methods is that a necessary transducermust be in a good optical contact with a person being examined. Withpersons in poor condition or in a cold environment, the bloodcirculation in a finger or an ear may be so weak that the requiredsignal cannot be produced. The light transmission variations caused byperipheral blood circulation do not include information about SV or CO.On the other hand, the CO meters based on electrical impedance variationrequire that several electrodes be attached to the body.

A person being examined in a ballistiocardiographic apparatus lies on aresponsive support and mechanical oscillations resulting from cardiacactivity are recorded. In principle, the SV and CO values can becalculated from the amplitude of oscillations. In practice, the resultshave shown a poor absolute accuracy but measurements at different timeson one person are relatively well reproducible. An inconvenience in thismethod is e.g. the provision of a sufficiently responsive bed for apatient in order to obtain sufficiently accurate measuring results.

It is further known to place a pad underneath a person being examinedfor producing a piezoelectric signal or some other electric quantity formeasuring a person's HR. Such measurements do not provide informationabout the CO or SV values of a person.

The prior known methods and equipment are neither capable of measuringHR, SV and CO values in a simple manner nor of producing simple indicesfor monitoring the condition of a person.

The present invention provides a solution for eliminating thedeficiencies of prior art and for designing equipment which provideinformation not only about HR but also about the SV and CO values andthe weight of a person. The quantities measured by means of methods andequipment of the invention can be used for producing index numbers formonitoring the progress of a person's condition. In order to achievethis, a method of the invention and apparatus based thereon arecharacterized by what is set forth in the characterizing sections of theannexed claims.

The most important benefits of the invention include simple operationand inexpensive design and particularly the information provided bymethods of the invention and corresponding apparatus about the physicalcondition of a person.

The invention is illustrated in the accompanying drawings, in which

FIGS. 1A-1C are schematic views of forces developing in the circulatorysystem as a result of cardiac activity,

FIG. 2 shows a block diagram for an apparatus of the invention,

FIGS. 3A and 3B show two different transducer designs,

FIG. 4 shows a block diagram for digital signal processing electronicsin an apparatus of the invention,

FIGS. 5A and 5B show one apparatus of the invention in a top and a sideview, respectively.

FIG. 1A illustrates schematically those parts of a human circulatorysystem that are essential in view of understanding the working of theinvention. The left ventricle of the heart H of a person P pumps on eachstroke a pulse of bloodflow into the aorta AO, which results in a changeof blood flow BF. The bloodstream turns in the aorta curve AC, which inturn produces a force FO directed towards the head of P as shown in FIG.1B. FIG. 1C is a graphic representation of the changes of BF and thecorresponding variations of weight G as a function of time t.

FIG. 2 illustrates an apparatus of the invention, wherein a person Pstands on a measuring support S, which is suspended on a frame F bymeans of sensing elements L, e.g. springs. Support S is fitted with atransforming element T, e.g. a strain gage transducer or apiezo-electric crystal or ceramics for recording the movements of Swhich depend on the weight of P and the compliance of L. It should benoted that a similar signal is obtained if a person is set in a sittingposition on S which is provided for example with a chair-like stand. Asitting position is feasible e.g. for poorly fit or handicapped personsor if long-term recording is desired. The output signal of T isamplified by means of an amplifier A, whose output is thus dependent onthe weight of P and can be shown on a display D1. The output signal of Ais carried to a derivator D, from whose output signal a pulse shaper Xproduces a pulse which corresponds to each fast weight change of P andwhich is forwarded to a central processor C, whose output provides HRshown on a display D2. X may also be provided with a detected-pulseindicator, a sound signal or a light signal or both. Another conceivablearrangement is such that the operator can select the type and intensityof an indicator signal at will or Switch it off completely. Integrationof the change of the output signal of A with an integrator I provides aresult proportional to stroke volume SV and multiplication of this withHR by a multiplier M provides a result proportional to cardiac outputCO, which is shown on a display D3.

Transducer T may comprise several separate transducers. Severaltransducers may be used e.g. for reducing the fluctuation caused by theswaying of a standing person. An apparatus intended for the examinationof a sitting person may include a chair-like measuring support andtherebelow four weight transducers, whose signals are added together.Furthermore, it may be beneficial to use a plurality of transducers suchthat a first transducer primarily records the weight of a person and asecond transducer records variations of the weight of a person. Thistype of transducer circuit is chematically illustrated in FIG. 3. In thecase of FIG. 3a, a force G' proportional to the weight of a patientapplies to a sensing element L which is fitted with a transformingelement Tw, possibly e.g. a strain gage transducer. Typically, this typeof transducer includes two elements, whose resistance changes in theopposite directions as sensing element L flexes as a result of force G'.These transducer elements are connected to an amplifier Aw, whose outputsignal Sw is proportional to force G'. The current bathroom scalesemploy a circuit, wherein the frequency of an oscillator is alteredunder the control of a transforming element Tw and this frequency isused for the derivation of weight. This type of solution, bothelectrical and mechanical, is included e.g. in the Hanson Hi-Tech(Bathroom scale, model 881) scale, manufactured by Hanson IndustriesLimited, Ireland. If such a circuit is to be maintained, it ispreferable to employ another transducer solution for recording thevariations of weight. Such other transducer solution is schematicallyshown in FIG. 3b. A transducer Tb is positioned e.g. on a measuringsupport S, such it will be subjected to the direct or indirect action ofa person's weight G. One preferred embodiment for transducer Tb involvesthe use of piezoelectric materials. Piezoelectric ceramic components aremanufactured e.g. by N. V. Philips Gloeilampenfabrieken, Eindhoven,Holland. Applications of these have been described e.g. in thepublication Piezoelectric Ceramic, Designer's Guide, Philips ComponentDivision 1989. Transducer Tb is preferably designed e.g. by using acomponent consisting of two piezoelectric ceramic elements(piezoelectric bimorph). An advantage offered by such element is arelatively low electrical and mechanical impedance, which makes itparticularly suitable for recording relatively low-frequency variables.The described-type of piezoelectric transducer can be included, as shownin FIG. 3a, in a flexible sensing element L and the obtained signal isforwarded to an amplifier Ab, whose output signal Sb is delivered tosignal processing electronics. Another way of using a piezoelectricmaterial is to provide support S with a layer of piezoelectric material,such as PVDF-film manufactured e.g. by Pennwalt Corp., PA, U.S.A andmarketed on the tradename Kynar Piezo Film.

FIG. 2 shows just one block diagram for an apparatus of the invention.For example, a quantity proportional to CO is obtained more accuratelyby averaging the values of SV and HR. Thus, the apparatus must beprovided with appropriate means for this purpose. In order to calculatean index representing physical condition or fitness, the above resultscan be converted e.g. into a digital form and a desired index can becalculated therefrom by means of a microprocessor. Thus, the apparatusmust be provided with appropriate means for this purpose. The requiredA/D converters are manufactured e.g. by Motorola Inc. and LinearTechnology Corp., U.S.A. and suitable microprocessors by Intel Corp.,U.S.A. The obtained result or results may be stored in a memory and usedlater as a reference in assessing the progress of a person's fitness.The recording means may comprise e.g. a semiconductor memory or amagnetic, optical or magneto-optical disc, a magnetic tape or aso-called smart card or even a punched card or tape. A suitablemicroprocessor assembly, which is compatible with so-called IBM PCequipment, is manufactured e.g. by Dover Electronics Manufacturing West,U.S.A. The apparatus is of type ESP8680 and the necessary interfaceequipment is available therefor. In mass production, it is of coursenecessary to design a solution for the apparatus, which is economical interms of production.

Instead of the embodiment shown in FIG. 2, the invention can beimplemented by utilizing the spectral information included in a signalcoming from the transducer. After transducer T and amplifier A, thesignal is converted to a digital form and it is transformed in tofrequency domain for example by using FFT algorith or so-calledautoregressive algorithms (Autoregreesive algorithms and other analyzingmethods, which are applicable when the number of samples picked up fromthe signal is small, have been described e.g. in the reference: Kay etal: Proceedings of IEEE, vol. 69, No. 11, 1981). This can be handilycarried out in said PC equipment but dedicated signal processingcircuits are also available. The resulting spectrum includes an intensepeak correlating to the heart rate (i.e. HR) and multiples thereof. Asimple program searches a suitable HR range for a maximum, which is HRat a given time. A suitable spectral range is 0.5-4 Hz, whichcorresponds to heart rates of 30-240 strokes/minute. This method isquick and not easily disturbed by the movements of a person. Theperiodicity of a signal can also be analyzed by using a correlationfunction. This type of solutions have been used in the analysis ofperiodical signals.

The aspects associated with spectral calculation and signal processinghave been studied in a number of publications, including for exampleProakis J. G. and Manolakis D. G.: Introduction to digital signalprocessing, Macmillan Publishing Company, New York, 1988 as well asOppenheim A. V. and Schafer R. W.: Discrete-Time Signal Processing,Prentice-Hall International Inc., Englewood Cliffs, N.J., 1989.

Several prior known solutions can be applied in the apparatus, whosemain principles have been described for example in the publicationTompkins W. J. and Webster J. G.: Design of microcomputer-based medicalinstrumentation, Prentice-Hall Inc., Englewood Cliffs, N.J., 1981. FIG.4 illustrates a block diagram for an apparatus wherein a signal STR,coming from a transducer and, if necessary, amplified in a preamplifier,is forwarded to a filter FBP, from there to an A/D converter ADC andthen to a processor DSP, whose output is advanced to a display DD.Filter FBP is a band pass filter which, when analyzing cardiac activity,eliminates the frequencies which do not include significant informationor which include disturbing signals produced e.g. by the swaying of aperson. The conducted tests have indicated that preferred frequencyrange is 6-15 Hz if the purpose is to record HR. It is preferred thatthe filter eliminates as effectively as possible low frequency (<1 Hz)components. The filter may be designed using the components and conceptsof analog electronics or at least some of the filtering may be effecteddigitally by means of processor DSP.

A number of prior known principles may be applied in transducersolutions. Since the quantity to be measured is a person's weight G,which is essentially a force produced by a person's mass and theacceleration of earth gravity, the changes of weight caused by cardiacactivity can be measured in a number of ways. The above description hasprimarily related to piezoelectric and piezoresistive transducerelements. It is prior known to measure variations of force by means ofsensing elements, such that transformations (number of transformations,rate of transformations) of sensing elements are measured by usingcapacitive, inductive, optical and acoustic methods. These principlesare described e.g. in publications: Allocca J. A. and Stuart A.:Transducers: Theory and Application, Reston Publishing Company, Inc.,Reston, 1984 as well as Cobbold R. S. C.: Transducers for BiomedicalMeasurements, John Wiley & Sons, New York, 1974. Combinations of priorknown transducer principles may be used for implementing apparatus ofthe invention in a variety of ways. The selection of transducerprinciples depends on several factors, such as manufacturing costs,accuracy requirements and power consumption. In the simplest, so-calledbathroom equipment, it is quite likely that the most preferredtransducers are piezoresistive or piezoelectric. When recording changesof weight, a piezoelectric transducer produces a powerful signal which,on the other hand, depends e.g. on the ambient temperature. However, thesolution may be preferable if the purpose is to record HR.

The results and calculated indices or a summary of results andcalculated indices can be printed on paper numerically and alsographically in the form of curves for thus supplying a person with adetailed representation of his or her progress. The apparatus can alsobe provided with signaling means, light or sound, which informs of thedetection of heartbeat. As mentioned above, these means may be coupledto the pulse shaper. The apparatus may be fitted with or it may beconnected to a necessary printing means, for example a matrix or laserprinter. Suitable printers are manufactured for example by Canon Inc.,Japan.

For example, the monitoring of physical fitness can be effected e.g. byusing indices R calculated as follows:

    R=c/(HR*G), or

    R=1/(HR*G),

wherein c is a number proportional to a person's SV or CO and derivedfrom the measuring results. It is preferred that the values used for SVand CO as well as HR are averages obtained during a period of severalheartbeats. As a long term result of physical conditioning, therest-measured HR of the heart decreases and SV increases quite rapidlybut the changes of weight generally occur slowly. As a person's physicalcondition improves, R calculated as above rises rapidly, this being amore responsive indicator of the direction of progress than mere bodyweight. Thus, R can be used as a support for weight-losing diets andfitness programs. An index indicating rapidly improving fitness improvesconsiderably motivation for training and dieting.

The above describes just one simple way of using measured quantities forcalculating an index indicating the progress or status of condition. Theindex formula may be adapted to include e.g. a target weight, wherebythe index progress and approach towards the target are clearlydiscernible. Also, different quantities may be given different emphasesdepending on the target of a training or weight-losing program.

Depending on the versatility of signal processing, the calculatedindices may be personal reference numbers, whereby the indices ofdifferent persons cannot be directly compared. If the signal processingis sufficiently versatile and the positioning of a person is handledcarefully, it is possible to derive indices and measuring results thatmay be used for the relative comparison of different persons. Thepositioning can be assisted by using e.g. a chair-like measuringsupport, whose back rest is tiltable for thus finding the maximal changeof weight caused by cardiac activity. The reason for this is that thedirection of aorta curve relative to the body is different in differentpersons.

The invention may be applied e.g. in domestic (bathroom) scales. In itssimplest form, such a scale would display the weight and HR of a person.For such a scale, it is possible to develop a custom-specific integratedcircuit. Separate displays are not required but HR and weight mayautomatically alternate on the display or the display may be selectede.g. with a separate switch, remote control or a the like. One such anapparatus is depicted on FIG. 5. FIG. 5a illustrates the top view of theapparatus. FIG. 5b is a side view of the apparatus. The measuringsupport S is in the frame F. The realization of the transducer meansaffects on the construction of the apparatus. Mechanically theconstruction of the apparatus may be similar than the construction ofthe above mentioned Hanson scale if e.g. strain gage transducer isutilized. In the display means DD there are indicators for HR and Wdisplay modes. In the HR mode the HR indicator is lighted up and thedisplay shows HR e.g. in beats per minute. Correspondingly, in the Wmode the display shows the weight of the person in kg:s or lbs and the Windicator is lighted up. The indicators may be e.g. light emittingdiodes (LED:s) or specific signs in a liquid crystal display. Onebenefit of the alternating display described above is the reduction ofmanufacturing costs. The alternating display saves also space and thefigures in the display may be designed to be of large size.

The display may also be fitted on a stand or be mechanically separatedfrom F. The communication between the display unit and the electronicsnear or in F may take place via wires, infrared light, (ultra)sound,capacitive or inductive coupling or radio waves. A benefit of this kindof remote display is that the person does not need to bow as he or shereads the display. The bowing movement changes the posture of the thoraxand affects on the values of the possibly measured quantities related toSV and CO.

The more elaborated devices of the invention calculate fitness indicesfor a person and may display quantities proportional to CO and SV. Suchdevices are useful for exercisers, athletes and may be available inprivate homes, health spas, gyms etc. Devices intended for public usemay be provided with a coin mechanism or a card reader for controllingthe use.

In more demanding use, more attention must be paid to the positioning ofa person being examined and a good solution is a chair-like measuringsupport, having more than one transducers (for example 3 or 4)therebelow. The support may be provided with a tiltable back rest aswell braces for setting a person repeatedly in the same position. Theback rest and other braces must be provided with scales or the like,whose readings are marked down or recorded in the memory of suchapparatus for future measurements. Such braces may be motorized and themotors may be controlled by the computer of the apparatus to measuringpositions e.g. on the basis of a personal identification code.Naturally, a method of the invention and corresponding devices may beused in many types of psycho-physical tests, in monitoring the treatmentof illnesses etc.

Monitoring SV and CO has been found beneficial when observing thetreatment of atrial fibrillation and the effect of a by-pass operation.Likewise, it is conceivable to use the changes of SV and CO forassessing the treatment of cardiac insufficiency. A method of theinvention and devices based thereon are capable performing this easilyand economically.

The above only describes a few embodiments of the invention. Theinvention can be subjected to a plurality of modifications within thescope of the inventive concept defined in appended claims.

I claim:
 1. A method for measuring quantities relating to a person'sphysical condition and cardiac activity, such as a person's weight andquantities proportional to heart rate, cardiac stroke volume, andcardiac output, characterized by:positioning a person on a measuringsupport so that a blood flow pulse caused by a stroke of the heart ismatched by a change in the weight; recording the person's weight andchanges in the weight; using time related characteristics of therecorded changes in the weight to determine information about theperson's cardiac activity including at least one of the quantitiesproportional to heart rate, cardiac stroke volume, and cardiac output;and, deriving said quantity related to the person's physical conditionfrom said weight and said at least one of the quantities proportional toheart rate, cardiac stroke volume, and cardiac output.
 2. A method asset forth in claim 1, further characterized by; using amplitude relatedcharacteristics of the recorded changes for deriving said informationabout said quantities related to the person's physical condition andcardiac activity.
 3. A method as set forth in claim 1 furthercharacterized by: calculating a frequency spectrum for the changes ofweight.
 4. A method as set forth in claim 1 further characterized by:calculating a correlation function for the changes of weight.
 5. Amethod as set forth in claim 1 further characterized by: calculating thequantities proportional to heart rate, stroke volume and cardiac outputby using quantities proportional to the time integral of the changes ofweight.
 6. A method as set forth in claim 5 further characterized inthat the quantities are utilized for calculating an index representingthe physical condition of a person that includes a correlation to theweight.
 7. A method as set forth in claim 6, characterized in that saidindex representing a person's physical condition includes a correlationto one of the heart rate and the cardiac stroke volume.
 8. An apparatusfor measuring variables relating to a person's physical condition andcardiac activity, such as a person's weight and quantities proportionalto the weight, heart rate, cardiac stroke volume, and cardiac output,characterized by:a measuring support for positioning a person thereon,so that a blood flow pulse caused by a stroke of the heart is matched bya change in the weight; a recording device for recording the person'sweight and changes in the weight; means for deriving the quantitiesrelated to the person's cardiac activity by utilizing time relatedcharacteristics of the recorded changes in the weight; and, means forderiving said quantity related to the person's physical condition fromsaid weight and said quantities related to the person's cardiacactivity.
 9. An apparatus as set forth in claim 8, characterized in thatit includes means for deriving said quantities related to the person'scardiac activity and physical condition by utilizing amplitude relatedcharacteristics of the recorded changes of weight.
 10. An apparatus asset forth in claim 9 further characterized by a transducer that producesone of electrical impedance changes and a voltage depending on theweight and its changes.
 11. An apparatus as set forth in claim 10further characterized by an amplifier and filter for using the outputsignal of the transducer to derive therefrom a signal proportional tothe changes of weight relating to the person's cardiac activity.
 12. Anapparatus as set forth in claim 11 further characterized by: means forcalculating quantities proportional to a time integral of the changes ofweight.
 13. An apparatus as set forth in claim 12 further characterizedby: means for calculating a frequency spectrum for the changes ofweight.
 14. An apparatus as set forth in claim 12 further characterizedby means for calculating an index representing the person's physicalcondition.
 15. An apparatus as set forth in claim 8 furthercharacterized by means for recording and displaying measured andcalculated results and calculated condition-represented indices.
 16. Anapparatus as set forth in claim 15 further characterized in that itincludes means for controlling use of the apparatus.
 17. An apparatus asset forth in claim 15 further characterized in that it includes asensory indicator that indicates the detection of a change of weight.18. An apparatus as set forth in claim 8 characterized in that saidmeasuring support is provided with a plurality of transducers.
 19. Anapparatus as set forth in claim 18 further characterized in that some ofthe transducers record the weight and some of the transducers record thechanges of weight.
 20. An apparatus as set forth in claim 8 furthercharacterized in that it includes a positioning mechanism for providingstandard and repeatable upright positions of the person's body on saidmeasuring support.